1. Field of the Invention
The present invention relates to a semiconductor device and fabrication method thereof.
2. Description of Related Art
In mounting a semiconductor chip on a package substrate or lead frame, a method is typically used which connects a surface (rear surface) on the side opposite from a surface (front surface) of the semiconductor chip element with the package substrate or lead frame by use of a mounting material. On the other hand, for CPU (Central Processing Unit) and the like particularly requiring a multiple-pin structure, miniaturization and slim structure, a flip chip mounting method is also used in which the front surface of a semiconductor chip, facing a package substrate or lead frame, is connected via projection-shaped bumps called “bump” with the package substrate or lead frame (for example, Patent Documents 1 and 2).
In the flip chip mounting method, solder bump or Au bump is used. Particularly, in a CPU for personal computer, high-melting lead solder bump is sometimes used. The reason for this is that, when a customer mounts the substrate, the solder of the flip chip mounting section is prevented from melting. In recent years, from a viewpoint of reducing environmental burden, there is increasing demand for lead-free (Pb-free) solder. However, an alternative cannot be supplied in the immediate future, so the bump of the flip chip mounting section is presently excepted from a list of lead-free soldering objects.
Similarly, high-melting lead solder is often used as a mounting material used to mount on a lead frame a semiconductor chip such as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) requiring low on-resistance, low thermal resistance, high reliability and the like. High-melting lead solder has excellent characteristics that excellent ductility is provided and thermal stress caused by temperature variation can be readily absorbed. Further, when a customer mounts the substrate, melting does not occur in a reflow process or the like, so high reliability is provided. Similarly to the high-melting lead solder bump for the flip chip mounting method, the mounting material for these semiconductor chips are also excepted from a list of lead-free soldering objects. However, from a viewpoint of reducing environmental burden, the customer's demand for lead-free solder used as the mounting material for a semiconductor chip has recently been rising.
Presently, conductive paste including silver flakes (Ag paste) is a lead-free material having a highest possibility of being used as the chip mounting material. However, Ag paste has an electrical resistivity three to five times greater than lead solder. Thus, it is difficult to use Ag paste in a product requiring low on-resistance.
Accordingly, as a result of diligent study on alternative lead-free material other than Ag paste, several proposals have been made (for example, Patent Document 3). FIG. 12A illustrates a top view of a structure of a transistor package described in Patent Document 3 as seen through encapsulating resin; and FIG. 12B illustrates a side view as seen through the encapsulating resin. In a semiconductor device 101, a semiconductor chip 102 is joined to the whole surface of a header 103B of a drain lead 103A with a lead-free solder 105 of the SnSbAgCu family having a solid phase temperature of 228° C., and a source electrode 123 and gate electrode 124 of the semiconductor chip 102 are connected via an aluminum wire 106 to a source lead 107A and gate lead 107B each being an external connection terminal. These parts are molded with an encapsulating resin 109. Here, a header 103B is exposed on the bottom surface of the semiconductor device 101, so the semiconductor device 101 can be connected to a wiring substrate.    [Patent Document 1] Japanese Patent Laid-Open No. 2008-28135    [Patent Document 2] Japanese Patent Laid-Open No. 2008-42077    [Patent Document 3] Japanese Patent Laid-Open No. 2005-340268